The scale on a map is 1cm : 9km. If the distance between two cities is 36km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 1cm on the map corresponds to an actual distance of 9km. An actual distance of 36km is the same as 4 $\cdot$ 9km. The distance between the two cities on the map is 4 $\cdot$ 1cm, or 4cm.